EP0958335B1 - Process for preventing high temperature corrosion - Google Patents

Abstract

An additive is sprayed into the combustion chambers of heating and waste incineration plants to prevent high temperature corrosion and to reduce the fly ash proportion in the exhaust gas. Also disclosed is a process for preventing high temperature corrosion and for reducing the fly ash proportion in the exhaust gas from heating and waste incineration plants. The disclosed additive consists of acid activated bentonite and/or oxide melts, in particular glass powder or glass dust with a melting point below 1000 DEG C, and is sprayed into the gas chamber in areas in which the gas temperature exceeds 700 DEG C.

Description

The invention relates to a method for preventing High temperature chlorine corrosion in combustion chambers of furnace and waste incineration plants and to reduce the Particulate matter in the combustion exhaust gases.

Known corrosion protection methods attempted to reduce the corrosion in combustion plants and in particular on the heat exchanger surfaces of the boilers by carrying out reactions to deactivate corrosive substances in the gas phase. For this purpose, magnesium oxide is generally injected, which means that high-temperature sulphate corrosion can be significantly reduced at temperatures above 480 ° C. This is achieved in that if there is an excess of MgO in the deposits which form, for example, on the superheater tubes, MgSO ₄ forms with the SO _{2 of} the flue gas and not alkali metal pyrosulfates. Alkali pyrosulfates dissolve the scale layer at temperatures above 480 ° C and lead to catastrophic corrosion damage.

Compared to corrosion damage caused by sulfation If chlorine is formed from chorides, MgO has no effect.

This type of corrosion has occurred increasingly in recent years The combustion chamber area of waste incineration plants as a result of the separation of waste has changed the waste composition on the one hand and on the other hand by legal regulations (at least 2 sec 800 ° C in the combustion chamber) a higher flue gas temperature is maintained must become.

From WO 95/11287 has already become known in the combustion chamber Cerium compounds such as iron cerium, cerium oxides and / or cerium oxide hydrates to spray in powder form. Such cerium compounds could use this older proposal together with magnesium oxide be injected and aimed primarily at the Cleaning effort for the removal of deposits on the inside of plant parts significantly, and in this way to allow a longer travel time than with known methods. In particular, passivating layers, which already superficially for the protection of plant upper parts have been trained, their effectiveness is maintained and are no longer affected. Cerium compounds such as Iron cerium, cerium oxides or cerium oxide hydrates should here in the Ensure near the walls reducing zones to this Way to reduce corrosion. The mode of action of the cerium compounds in the manner of oxidation catalysts leads to afterburning in reducing areas of the flue gas, whereby a reduction of one already trained for passivation Oxide layer of scale or rust is definitely avoided becomes. The cerium compounds should also protect against chloride ion corrosion take effect, and in this way oxidation to principally less dangerous chlorine gas in molecular Ensure shape.

However, the use of such additives is relatively high Related costs and has been particularly related to the high temperature chlorine corrosion, in which chlorine gases in turn dissociate, not proven to be sufficiently effective.

Experiments, filter aids and especially inorganic Filter aids based on silanol groups to mix active silica with the feed to be burned, have also proven to be relatively complex, because of the inhomogeneous distribution in the garbage additives Only show effect in the percentage range.

For the return of contaminated residues such as for example filter ash, adsorbents or the like. in the cycle a furnace with the addition of waste glass and / or Lime chippings have already been proposed in DE-A-4 021 362. the absorber in the circuit of a melting furnace full ash return. The absorber can thereby in the flue gas path and / or in the ash return and or fed into the slag discharge and / or with the coal be used, with used glass and / or lime chippings being metered in can be.

GB-A-1 307 127 discloses the use of 85% by weight calcium bentonite, 10% by weight sodium phosphate and 5% by weight sodium borate in oil, gas and coal furnaces as known. US Pat. No. 3,249,075 proposes the introduction of antimony compounds and silicates with a large specific surface area in the superheater area of coal-fired boilers to reduce SO ₃ corrosion.

Finally, GB-A-800 445 proposes burned bentonite and other refractory materials for better Use distribution of oil drops in gas turbines.

The invention now aims at a method of the beginning to create the type mentioned, in which even the smallest quantities of an additive can be used, alkali and The content of metal chlorides in the flue gas reduced as quickly as possible can be. The reaction should be as quick as possible use and also take place at high temperatures without elemental chlorine is released.

To achieve this object, the method according to the invention for reducing high-temperature chlorine corrosion in firing and combustion plants, in which additives are injected into the gas space, essentially consists in the fact that acid-activated bentonite in the gas space at gas temperatures above 750 ° C., preferably 800 ° C. is injected. Due to the high reactivity of acidically activated bentonite, the desired reaction for setting alkalis can be carried out even at relatively high temperatures, the desired reactions already taking place quickly and quantitatively even at temperatures of over 900 ° C. Alkali can be set very early if the additive is injected together with secondary air just above the burner level or the grate, for example in the level of the secondary air supply, so that the distance over which a corrosive attack is at all possible can be significantly reduced. For better distribution of the acid-activated bentonite, this can optionally be injected together with silicon dioxide, with SiO ₂ acting primarily as a diluent. In any case, more than 50% by weight, based on the mixture, of acid-activated bentonite should be used in mixtures with SiO ₂ in order to ensure the desired reaction quickly.

By injecting glass dust it can be ensured that the glass dust or glass powder quickly melts completely, being an effective protection of those to be protected against corrosion Boiler walls can be improved in that the glass dust or the glass powder over directed nozzles or spray plates in Direction of the walls of the gas space is introduced. To this A flushing flow of the glass melt along the Walls of the gas room ensured.

According to the invention, glass dust or glass powder is advantageously used in Quantities of 0.3 kg / t to 30 kg / t of waste are injected into the gas space, where ground waste glass, e.g. Window glass or bottle glass, can be used. For the invention The process is advantageously based on grain sizes about 50 microns of ground glass used.

It is advantageously carried out in such a way that acid-activated bentonite, if appropriate mixed with SiO _{2, is} injected into waste incineration plants in quantities of 0.5 to 3.0 kg / t of waste.

A particularly economical process management among others Pollutant emissions can be reduced by that acid-activated bentonite for adsorbing pollutants, such as mercury or dioxin in the cooled exhaust gases from combustion plants, used in particular in an entrained current flow is then sprayed into the combustion chamber becomes.

Due to the prevailing conditions in the combustion chamber, a not inconsiderable amount of alkali metal chloride is formed at the high chlorine content that is usually present in waste today. The ratio of chlorine to sulfur in waste has shifted in flue gas in favor of chlorine in recent years, with increased amounts of metal chlorides entering the flue gas without decomposition and only being converted into sulfates there or in the deposits. Under the given thermodynamic conditions, this reaction, which is also called the sulfation reaction, leads to sodium sulfate and elemental chlorine, and thus to a strong corrosive attack. The chlorine reaches the pipe surface and can destroy steel with the formation of iron chloride. This sulfation reaction can be observed primarily in the combustion chamber behind and just above the brick lining, and the injection of acid-activated bentonite according to the invention enables the content of metal chlorides to be reduced extremely rapidly. Proposals to inject sulfur into the flue gas for this purpose lead to an acceleration of the sulfation of the chlorides, but the sulfation only takes place at lower temperatures and the amount of chlorine released remains the same. Due to its chemical and physical properties, the acid-activated bentonite is able to react very quickly with alkali compounds in the flue gas even at higher temperatures, especially at temperatures above 900 ° C, whereby alkali can be set and HCl is formed. No elemental chlorine is released, which significantly reduces the risk of high-temperature chlorine corrosion.

An inexpensive and simple additive that can also be used for showering in the combustion rooms of combustion and waste incineration plants, with which the travel time of Kessein can be increased significantly and which at the same time still aims relative to the known additives to drastically reduce the high level of dust in the exhaust gas according to the invention from oxide melts, in particular glass powder or Glass dust obtained with a melting point below 1000 ° C. Glass powder or glass dust, which preferably in counterflow to the When flue gases are injected, it melts at the temperatures in the combustion chambers, taking flight dust too water-soluble glass is bound. At the same time surprisingly in such melting glass flour or Glass dust particles also set alkali quickly, surprisingly also a rapid decrease in the chlorine content in the exhaust gases was detected. All of these corrosive components the combustion gases are thus from the melting ones Glass powder or glass dust particles set effectively, whereby to all abundance the advantage is achieved that the melt at the Wall of the boilers a dense and a corrosive attack preventive coating which forms as a liquid melt film long ago the walls flow towards the slag. Such Flushing the boiler walls through the molten glass melt thus has in addition to the corrosion-reducing effect the boiler walls to be protected have the advantage that a number of Effective discharge of pollutants with the downward flowing melt can be. This additional additive can be common be injected with acid activated bentonite, whereby the effects even improve over the sum of the individual effects to let.

Advantageously, glass powder or glass dust with one Melting point below 800 ° C used, a complete Melting and safe removal of the in the Melt dissolved pollutants can be ensured can that the glass powder or the glass dust a medium grain size from 30 to 60 µm, preferably from 40 µm.

The invention is explained in more detail below with the aid of two diagrams. 1 of the drawing shows the amount of sodium chloride and the SO ₂ equilibrium partial pressure over the temperature for the subsequent furnace conditions. pCO2 = 0.2 bar, pH ₂ O = 0.2 bar, pO2 = 0.05 bar, pHCl = 10 ^-5 bar and pCl = 10 ^-5 bar. 1 that the formation of sodium chloride prevails in the combustion chamber, particularly in the high temperature range, and the aim is therefore to eliminate sodium chloride from the flue gas even at high temperatures.

2 shows the amount of sodium chloride in mg / m ³ over the temperature. Curve 1 shows the reduction that can be achieved by injecting 1 kg of acid-activated bentonite per t of waste. Curve 2 shows the effect which could be achieved by injecting 2 kg of sulfur, so that the superiority of injecting acid-activated bentonite for the purpose of reducing the sodium chloride content at high temperatures is clearly evident. Curve 3 illustrates the effect when only 1 kg of sulfur is injected, and curve 4 the effect with pure flue gas.

The additive is powdery and can easily be added in one Grinding fineness can be produced, which allows the additive to be injected directly into the combustion chamber via the secondary air.

When glass was injected at the same time, it became corrosion-resistant Lining of the boiler is formed and the pollutant emissions reduced, the service life of that of the combustion chamber downstream device, such as superheater or the like, improved has been.

Claims (5)

1. A method for preventing high-temperature corrosion and reducing the portion of flue dust contained in the combustion flue gases of furnace arrangements and waste incineration plants, characterized in that acidically activated bentonite is nozzled into the gas space at gas temperatures of above 750°C, preferably 800°C.
2. A method according to claim 1, characterized in that acidically activated bentonite is nozzled in in a mixture with SiO₂, the portion of said acidically activated bentonite being larger than 50 wt.-%, based on said mixture.
3. A method according to claim 1 or 2, characterized in that acidically activated bentonite optionally mixed with SiO₂ is nozzled into waste incineration plants in amounts of from 0.5 to 3.0 kg/ton waste.
4. A method according to claim 1, 2 or 3, characterized in that ground glass wastes such as, e.g., window glass or bottle glass wastes are used as said SiO₂ component.
5. A method according to any one of claims 1 to 4, characterized in that acidically activated bentonite is used for adsorbing noxious substances such as Hg or dioxin contained in the cooled flue gases of combustion plants, in particular by way of a flue flow process, and subsequently is nozzled into the combustion space.

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